This invention relates generally to semiconductor chip package assembly, and in particular to flip chip package assembly. More specifically, the invention relates to a consolidated flip chip BGA assembly process and apparatus in semiconductor device assembly.
Semiconductor flip chips can be bonded directly to a packaging substrate, without the need for separate wire bonds or tapes. Flip chip dies normally have solder bumps affixed to their bonding pads. During a packaging process, the die is “flipped” so the solder bumps form electrical interconnections between the die and metal pads on a packaging substrate. Semiconductor dies (chips) of this type are commonly called “flip chips.” Flip chip technology has several advantages over earlier wire bond technology for many applications. Flip chip technology offers the highest speed electrical performance of any semiconductor package technology, since eliminating bond wires reduces the inductance and capacitance of the die to package substrate interconnection and greatly shortens the interconnection path. The replacement of perimeter wires with direct solder bonds on the active face of the chip also results in a more durable package with greater input/output connection flexibility.
Despite all the advantages of flip chip technology, the microelectronic assembly process used to package a flip chip die for connection to an electronic device remains a time consuming multi-stage process.
Referring to FIG. 1, stages in a conventional method 100 for packaging a semiconductor flip chip are illustrated. First, a flip chip die and a packaging substrate are electrically interconnected and mechanically bonded in a solder joining operation. The die 10 is aligned with and placed onto a packaging substrate 104 such that the die's solder bumps 106 are aligned with metal pads or pre-solder on the substrate 104 (101). The substrate 104 can be made of an organic metal oxide or ceramic material, for example. Heat is applied causing the solder bumps to reflow and form electrical connections between the die and the packaging substrate (110). The package is then cooled to harden the solder jointed interconnection.
An underfill 122 is then applied in order to enhance the mechanical bonding of the die and substrate (120). An underfill material, typically a thermo-set epoxy, is dispensed to fill the remaining space (or “gap”) between the die and the substrate. The underfill is then cured by a heating process (130).
In order to improve the thermal performance and reliability of flip chip packages, stiffeners and/or heat spreaders are often used. A stiffener 142 may be placed around the die on the substrate where it is bonded with a heat curable organic adhesive. The stiffener (also sometimes referred to as a “picture frame”) is typically a flat piece of high modulus metal about 10 to 40 mils thick, having substantially the same dimensions as the package substrate with a window in its center to clear the die. Typically, the stiffener is composed of nickel-plated copper which has a coefficient of thermal expansion similar to that of typical substrate materials. The stiffener is typically bonded in a separate step following curing of the underfill material (140). The purpose of the stiffener is to constrain the substrate in order to prevent its warpage or other movement relative to the die which may be caused by thermal cycling during operation of an electronic device in which the package is installed. Such movement may result from the different coefficients of thermal expansion (CTE) of the die and substrate materials, and may produce stress in the die or the package as a whole which can result in electrical and mechanical failures.
A heat spreader 162 (a heat sink also sometimes referred to as a “lid”), typically composed of a high thermal conductivity material, and having substantially the same dimensions as the package substrate is typically also attached over the stiffener 142 and the die 102 and bonded to the substrate by a thermally conductive organic adhesive 152. The heat spreader may have a hard or a soft connection with the die via a thermal compound, typically a thermal adhesive or grease, respectively, applied to the top of the die 102 (on its inactive surface) and the stiffener 142 (150). The same or different thermal compound/adhesive can be used for bonding to the die and the stiffener. A conventional heat spreader is also typically a flat piece of the same type of material that is used for the stiffener, for example, nickel-plated copper about 20 to 40 mils thick. A heat spreader may also have a form that allows for direct attachment to the substrate, such as through edges or legs that descend from the flat piece overlying the die to contact the substrate. The heat spreader 162 is usually applied in a separate step following attachment of the die 102 and stiffener 142, if any (160). The purpose of the heat spreader is to disperse the heat generated during operation.
To complete the flip chip package 172, solder balls 174 are attached to the opposite side of the packaging substrate 104 from the die 102 to form a ball grid array (BGA) used to attach the flip chip package to a printed circuit board (PCB) for incorporation into an electronic device, in a subsequent process.
This packaging technique requires several pieces of equipment, multiple process steps and take several days to complete the assembly process. The complexity and time required result in substantial cost. Thus, it would be desirable to simplify and shorten the flip chip packaging process.